There's a lot of talk about politics becoming more divisive across parties, with less and less common ground, and Republicans being called and embracing the "party of no" label. But is it true? No need to speculate; we can find out with the help of the raw data in the form of Senate roll call votes (the last ~21.5 years of which are conveniently available), plus some perl scripts.

Number of votes over time

Let's start with something simple. Here's the number of yea/nay votes over time in the Senate. (By yea/nay votes, I mean I'm excluding some exceptional votes such as when the vote is Guilty/Not Guilty.) As the plot below shows, the number of votes has remained fairly steady across time, except that odd-numbered years get more votes and there was an unprecedented spike in 1995. And of course, 2010 is at a disadvantage for obvious reasons.

The Party of No

Does one party vote "no" more often? Below is the fraction of Yea votes cast by each party across time. There is some signal in this data, such as the Republican takeover in the mid-90s. And 2010 is so far at very low fraction of Yea votes by Republicans (49%), beat only by 1993 (43%). But this data clearly requires some more interpretation as to what a Yea or Nay vote actually means. One has to question, for example, why Republicans voted Yea more often in 2009 than Democrats.

The naysayingest senator

Moving to individual member stats, aggregated over the 21.5 years of the data, most members vote no about 30-40% of the time, but there are outliers who are quite agreeable and quite disagreeable:

Here are the top 20 outliers on either end. Current sitting senators, none of whom are in the Yeasayingest column, are linked to their Wikipedia pages.

Jim DeMint takes the prize as the naysayingest sitting senator, but he's Walloped by the naysayingest of all time who voted no on almost half his votes. Malcolm Wallop very nearly maximized the entropy of his votes. Again, this data should be taken with a grain of salt, since how often one votes Nay depends on who currently controls congress.

*Barkley and Goodwin don't really count; Barkley was briefly appointed to replace Paul Wellstone and only cast 14 votes, 12 of them Yea. Goodwin was appointed less than two weeks ago to replace Robert Byrd and has only cast 11 votes, 6 of them Yea.

General disagreement

Yea or Nay votes could mean almost anything, depending what question is being voted upon. Here's a more robust metric: the agreement of a vote is 1 if everyone voted the same way, 0 if the vote was split 50/50, and linearly interpolated in between. We're currently at an all-time low of 34.4% (where "all-time" = the last 21.5 years), as you can see below.

But this doesn't expose the divisiveness between parties.

Political divisiveness

Here's what we really want to see: the average distance between Republicans and Democrats. On some particular vote, we can represent the average Democrat position as the fraction of Dems voting Yea; same for Republicans. Divisiveness is the distance between these two average positions. For example, if 50% of Dems and 50% of Reps vote yes, then divisiveness is 0. If 10% of Dems and 90% of Reps vote yes (or vice versa), then divisiveness is 0.8.

The data shows a striking difference. Politics were more centrist in the late 80s. Divisiveness didn't move much for about 18 years, but then divisiveness dramatically spiked since the beginning of the Obama administration, setting a record in 2009 and another record so far in 2010. The difference here is really quite dramatic: 29% divisiveness in 1989, vs. 70% today.

Party unity

Party unity is closely related to divisiveness. I'm defining unity as the fraction of members who take the majority position in their party, so 0.5 is the minimum score and 1 is the maximum. Party unity appears to have increased over time, though with some wild shifts particularly on the Republican side. Interestingly, Democrats seem to be just as unified as Republicans on the mean vote.

Anyone know where to get data going back earlier than 1989?

Update (July 29, 2010):

A few important points to emphasize:

One shouldn't read too much into this data. Divisiveness, as defined above, has increased---but this says nothing about why it has increased. As commenter GoldenBoy pointed out at politicalwire.com, we should naturally expect divisiveness to increase when one party has control of congress and the White House, since they have less need to work with the other party in order to pass legislation. The analysis here really says nothing about to what extent this or other factors caused the increased divisiveness.

It's important to put the recent increase in divisiveness in the context of a longer-term trend of increasing polarization. There does seem to be a divisiveness spike in the last couple years, but in general an increase is not surprising.